Solar photovoltaic modules, called “photovoltaics” or “PV”, are solid-state semiconductor devices with no moving parts that convert sunlight into direct-current electricity. Crystalline silicon (“Si”), including mono-crystalline and multi-crystalline Si, accounts for about 85% of the annual PV shipments. In the last few years, multi-crystalline Si(“mc-Si”), which can be grown by low-cost methods such as the casting method, has replaced mono-crystalline Si as the most dominant starting material for solar cells.
Although mc-Si has an advantage over mono-crystalline-Si in that it can be grown through cheaper, low-cost methods such as the casting method, there is a disadvantage to mc-Si. Specifically, the conversion efficiency of solar cells based on mc-Si is less than the conversion efficiency of solar cells based on mono-crystalline Si. It would be ideal to improve the conversion efficiency of mc-Si utilizing current processing technologies, without drastically increasing the production cost.
The prior art has recently proposed multi-crystalline SiGe with microscopic compositional distribution as a novel PV material. The fundamental idea is to non-uniformly disperse a small amount of Ge in mc-Si for controlling macroscopic physical properties, which are important for solar cells. In contrast to SiGe with uniform composition, which has been intensively studied for electronic applications, physical properties of mc-SiGe can be widely tuned by controlling the microscopic compositional distribution while fixing the average composition or concentration. United States Patent Publication No. 2002/0139416 to Nakajima et al. relates to such a multi-element poly-crystal having a non-uniform microscopic distribution that uses silicon and germanium, and this publication is herein incorporated in its entirety by reference.
Ideally, it would be possible to extend the onset of the absorption to the bandgap of Ge if pure Ge can be dispersed in mc-Si. If built-in strain is utilized, further reduction of the bandgap is expected to be possible. As a consequence, a drastic increase of the photocurrent due to the increased absorption of near-infrared light is expected. However, a drop of the open-circuit voltage cannot be avoided due to the increase of the intrinsic carrier concentration, as a result of the smaller bandgap of SiGe compared to that of Si.
Thus, there exists a need for further improvements in the field of solar cells, and in particular to solar cells made of mc-SiGe. The present invention solves this need by providing an improved solar cell via control of the average concentration of Ge in the multi-crystalline Si—Ge material.